(1) Field of the Invention
This invention relates to microwave antenna systems.
(2) Prior Art
Various types of microwave antenna systems are known. For example, a center fed J-hook support structure is known to provide a mechanically simple feed support for a reflector antenna. However, such an antenna may not have a desirably small size and low sidelobe radition pattern.
Referring to FIG. 1, there is shown a known antenna system, such as that disclosed in U.S. Pat. No. 2,534,271 for an antenna system issued Dec. 19, 1952 to Kienow. This patent teaches an antenna system wherein the J-hook feed device is attached to the edge of a portion of a parabolic reflecting surface. The reflecting surface is not centered about the axis of surface revolution (i.e., the axis of symmetry) and the feed horn does not point along the axis of surface revolution to the reflective surface. That is, the reflector used in the Kienow antenna system is an offset reflector surface which is formed by an off-center section of a paraboloid. The stated aim or object of the Kienow patent is to provide increased directional accuracy. However, the system does not provide for a desired manipulation of the sidelobes so as to produce a particular antenna radiation pattern. Typically, it is desirable to substantially reduce or eliminate the present of sidelobe. In particular, the Kienow microwave antenna system includes a circular off-center section of a paraboloid and an electromagnetic horn disposed substantially at the focal point of the paraboloid. The line corresponding to the direction of maximum response of the horn intersects the surface of the section at a point equal to eliminate the edges of the section.
The problem of reducing the sidelobe radiation pattern is better understood by an appreciation that the magnitude of sidelobe energy radiated by a reflector antenna is controlled by several different contributors. These can be divided into three major categories which depend on the source of sidelobe energy: direct feed illumination and spillover; support strut scattering; and mechanical tolerance effects. The first and third categories are present in all reflector-type antenna systems; the second, support structure scattering, is present only in symmetrical, front-fed antennas. In order to successfully describe the complete sidelobe envelope, each of these factors must be evaluated and vectorally summed to provide the total field at a given point in space.